Main optical characteristics

  • 1500mm free aperture
  • Gregory configuration with additional tertiary mirror (M3)
  • light weighted  optics
  • integrated adaptive optics
  • Image de-rotatornominal field of view 150" (max. 300")
  • effective focal length: 55.6m (F/38) 
  • low instrumental polarisation
  • polarisation and calibration unit in symmetric beam
  • wavelength  range from  350nm to several µm
  • night time observations  possible
  • mirrors made from silicon carbide (Cesic) 
  • primary mirror (D=1,5m) active thermally controlled 
  • M2 (D=0.43m) and M3 (D=0,36m) passive cooled

The telescope uses a 3-mirror Gregorian configuration with three active mirrors. The primary mirror (f/1.7) is thermally controlled (with 200W absorbed power the temperature difference to ambient air shall not exceed 0.5°C). light weighted and thermally controlled primary mirror (M1). A cooled field stop at the prime focus F1 provides a field of view of nominal 150arcsec (maximum 300 arcsec) and reflects the unused light outside the telescope. The elliptical secondary mirror M2 (F1/1.29) magnifies the primary image and generates the secondary focus (F2) 200 mm above of the elevation axis. A polarimetry package is located near the secondary focal plane F2, at the center of the tube. An elliptical tertiary mirror M3 (F/3.97) reimages the secondary focus via M4 and through the coudé train (M5, M6, M7) into the laboratory. M3 is supported by an axial drive stage which is used for focusing at the tertiary focus.

The image rotation induced by the alt-azimuthal mount is compensated by a rotating image de-rotator with three mirrors. The de-rotator could be removed from the beam.

A flat mirror M11 redirects the beam horizontally into the laboratory feeding the adaptive optics (AO) system. M11 and the following parts rest on an optical table in the observing room. For use of the telescope without AO M11 can be removed. The scientific focus will be distributed by a rotating mirror to the different post focus instruments. 

Mirrors

Originally it was planned to manufacture the first three mirrors of the GREGOR telescope from the light-weight silicon carbide material Cesic.  For the primary this would have resulted in a weight of only 90 kg.  Cesic has a very high thermal conductivity - more than 100 times better than the glass ceramic materials with low thermal expansion that are normally used in astrophysics.  This results in a very homogeneous mirror temperature, thus facilitating the cooling of the reflecting surface.  When the primary is directed towards the Sun, it absorbs some 200 W, heating up the surrounding air unless the 
mirror surface is efficiently cooled.  This would cause dramatic losses in image quality.  Using an appropriate cooling system, the temperature of the GREGOR primary will therefore be stabilized to temperature differences smaller than 0.5°C w.r.t. the ambient air. 
Unfortunately, technological problems prevented the manufacture of the 1.5 m primary from Cesic material.  Therefore, a lightweighted main mirror from Zerodur was manufactured.  The mass of the mirror is about 215kg. Cooling and mounting of the mirror were adjusted to the new material in order to achieve a comparable performance. 
For testing purposes, a 1 m Cesic mirror could be used temporally. 
The secondary and tertiary mirrors are made from Cesic and ready for active cooling. 

Adaptive Optics

The GREGOR telescope is equipped with a high order adaptive optics (HOAO). It will allow diffraction limited resolution (0.08‘‘) for seeing above r>= 10cm. The wavefront sensor has156 subapertures to measure the wavefront deformation. The deformable mirror has 256 actuators and is able to correct for 140 degrees of freedom with a control loop frequency of 2000Hz.

In the near future an extension called multi-conjugate adaptive optics will extend the corrected field by a factor of more than 10.